Screw compressor-expander cryogenic system with magnetic coupling

ABSTRACT

Cryogenic refrigeration system 10 employs a screw compressor 72 and a screw expander 70 wherein one rotor of the compressor and one rotor of the expander is mechanically driven, including a magnetic coupling 50, 52 between the compressor and expander rotors. The other compressor rotor and other expander rotor is driven only by the connected rotor.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation in part of patent application Ser.No. 894,677 filed Apr. 10, 1978 by Bruno S. Leo for "ScrewCompressor-Expander Cryogenic System", the entire disclosure which isincorporated herein by this reference. That application was abandonedand a continuation thereof was filed on June 12, 1980. The continuationapplication received Ser. No. 158,764 and is now U.S. Pat. No. 4,291,547issued Sept. 29, 1981.

BACKGROUND OF THE INVENTION

This invention is directed to a cryogenic system where both thecompressor and expander, which operate with the cryogenic refrigerantfluid in the system, are rotary screw-type machinery of the Lysholm typewith the expander being coupled to the drive system with a magneticcoupling operating through a wall to separate the compressor andexpander chambers.

Lysholm built an early prototype of the rotary screw compressor in 1934.Some of his development work was described in the Proceedings of theInstitution of Mechanical Engineers, Vol. 150, No. 1, Pages 11-16 and 4plates 1943. One of the main features of this screw compressor is thefact that it can run without oil or other lubricant in the compressionchamber. No oil is necessary because the rotors do not contact eachother or the casing. The only mechanical contact is in the bearings andin the timing gears (if any) which can be located on the outside of thegas containing casing and away from the refrigerant gas flow stream.

The Lysholm type rotary screw compressor has two rotors withintermeshing lobes. Within the intermesh of the lobes and housing, thecompression takes place. Two helical rotors comprise the working partsof the screw compressor. The male rotor generally has four lobes androtates 50 percent faster than the female rotor which has six flutesbetween which are grooves in which the lobes interengage. Other ratiosof lobes to flutes are also used. The gas is compressed in the spacesbetween the housing, the lobes and the grooves. The lobes and thegrooves are helical so that the space appears to move progressivelytoward the outlet end of the housing, and the space becomesprogressively smaller along the length of the rotors as the rotorsrotate. Thus, gas taken in the inlet port at the suction end iscompressed in the space as the rotors turn and the gas is delivered athigher pressure from the outlet port at the delivery end of the housing.The inlet and outlet ports are automatically covered and uncovered bythe shaped ends of the rotors as they turn.

There has been considerable development work done on the improvement ofsuch screw compressors. Most of the patents are owned by Svenska RotorMaskiner which devoted the pioneer effort in this art and appears tohold most of the patents. The company is located in Nacka, Sweden.

Nilsson, U.S. Pat. No. 3,245,612 and Schibbye, U.S. Pat. Nos. 3,283,996and 3,423,017 are particularly directed to the shapes of the lands andthe grooves in the rotors, but show the porting and general organizationof the rotary screw compressor to show how compression and expansion areachieved in such a structure. Furthermore, this type of screw compressoris illustrated as being the compressor in refrigerator systems in U.S.Pat. Nos. 3,432,089; 3,811,291; 3,848,422 and 3,945,216. While the useof screw compressors has been recognized for refrigerator compressorservice, the use of such devices as expanders for such service has notbeen recognized. Furthermore, it has not been previously recognized thatscrew compressors and expanders in the same refrigerator can efficientlyrun at about the same speed so that they can be coupled directly orthrough gearing, for speed control of the expander and for powerfeedback from the expander. In the refrigeration arts, it is known thatwith source gases and conditions it is necessary to extract work duringexpansion to produce refrigeration, with some refrigerant gases withinsome of their operating temperature ranges. In the past, pistonexpanders have been used, usually in smaller refrigerators, and turboexpanders have been used, usually in large refrigerators. While the workoutput of such expanders is not significant in terms of totalrefrigeration input power, speed control of the expander is necessary.Such speed control has been difficult where the turbo expander runs atvery high speed. It is part of this invention that the employment of anexpander coupled to and running with the compressor is feasible whenscrew-type equipment is used for refrigerant gas compression andexpansion.

In order to supply economical refrigeration a design of minimumcomplexity, associated with long and trouble free life, is necessary.The structure of this invention provides a refrigerator which is of lowweight per unit of refrigeration, and is especially designed so that inthe small sizes for which this refrigeration system is most suitable,the structure is of simplified mechanical design. Thus, such arefrigeration system can be used to cool devices for long maintenancefree life and can be employed in locations where total weight and inputpower should be minimized.

SUMMARY OF THE INVENTION

In order to aid in the understanding of this invention, it can be statedin essentially summary form that it is directed to a screwcompressor-expander cryogenic refrigerator system wherein a wall ispositioned between the compressor and expander housings, with magneticdrive through the wall to eliminate flow of gas between the chambers.Furthermore, the rotors of both the compressor and expander are designedso that they can operate without timing gears.

In addition, valving can be provided to employ the regenerator systeminstead of a counterflow heat exchanger system in the screwcompressor-expander cryogenic system of this invention, and theregenerator system as illustrated in connection with a two expanderstage machine.

It is thus an object of this invention to provide a screwcompressor-expander cryogenic system wherein Lysholm-type gas handlingequipment is used for compression and expansion of the gas to producerefrigeration. It is another object of this invention to provide arefrigerator wherein a long trouble-free life is achieved. It is afurther object to provide a screw compressor-expander cryogenicrefrigerator system wherein the expander runs at the same relativerotative rate as the compressor to permit coupling between the expanderand compressor for speed control of the expander and feedback of workfrom the expander to the compressor.

It is another object to provide a screw compressor-expander cryogenicrefrigerator system wherein a wall is provided between the compressorchamber and the expander chamber and magnetic coupling provides therotary mechanical coupling between the rotors in the two chambers toprevent gas leakage through the rotor bearings. It is a further objectto provide a screw compressor-expander cryogenic refrigerator systemwherein the two rotors in the compressor chamber are coupled togetherthrough a hydrodynamic gas film so that the driven rotor rotates withoutmechanical contact against the driving rotor, and in the expanderchamber, the rotors are not geared together but are separated byhydrodynamic gas action to reduce wear on the running parts of therefrigerator system and this maintain a long life.

It is another object to provide a screw compressor-expander cryogenicrefrigerator system wherein the employment of a screw-type compressorand a screw-type expander permits the production of refrigeration at anincreased value of unit of refrigeration per unit of weight so that thesystem can be employed in locations where weight is critical. It is afurther object to provide a cryogenic refrigerator system which employsa screw-type compressor and a screw-type expander wherein morerefrigeration is produced per unit of input power to permit use of therefrigerator system in locations where power must be conserved.

It is a further object to provide a screw expander-compressorrefrigeration system wherein a multiple stage expander is employed toincrease system efficiency and provide two temperature levels ofrefrigeration. It is another object to provide a screwcompressor-expander cryogenic refrigerator system wherein valving isdriven by the rotors of the compressor-expander equipment and thevalving controls gas flow through regenerators for exchanging heatbetween flow streams.

Other objects and advantages of the this invention will become apparentfrom a study of the following portion of the specification, the claimsand the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevational view of the screw compression-expandercryogenic system of this invention with parts broken away and partstaken in section to show the cryogenic gas flow path through thecompression and the expansion chambers and through the heat exchangers.

FIG. 2 is a side elevational view of a similar screw compressor-expandercryogenic system showing the machinery as interconnected by alubrication system.

FIG. 3 is a longitudinal section taken through the center line of ascrew compressor-expander cryogenic system showing the details of thecompression and the expansion chambers, the magnetic couplingtransmitting power and speed control between the compressor and theexpander rotors and the lubrication of the various bearings.

FIG. 4 is a longitudinal section through another preferred embodiment ofthe screw compressor-expander cryogenic system of this invention withmagnetic coupling between the stages, with multiple expander stages andwith valving for reversing flow in external passages so that it can beused in conjunction with heat exchange regenerators.

FIG. 5 is a longitudinal section through a valve suitable for flowreversing.

FIG. 6 is an elevational view of the valve face, as seen generally alongthe line 6--6 of FIG. 5. FIG. 7 is a schematic flow diagram showing thescrew compressor-expander cryogenic system with two expander stages andwith heat exchange regenerators.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The mechanical equipment 10 of the screw compressor-expander cryogenicsystem is generally indicated in FIGS. 1, 2 and 3. In FIG. 1 it is shownin association with the external gas flow equipment 12 which completesthe cryogenic system and in FIG. 2 it is shown in association with thelubrication plumbing 14 which is external to the machinery 10. Themachinery 10 comprises drive motor 16 which is usually an electric motorand is positioned within motor housing 18. Motor housing 18 is dividedby wall 20 so that the motor bearings 22 and 24 in bearing bosses 26 and28 are in separate lubrication compartments.

End wall 30 serves as the end wall for compressor chamber 32 as well asmounting boss for the compressor drive end bearings 34 and 36. Wall 38through motor housing 18 separates the compressor drive end bearings 34and 36 from motor drive end bearing 24 to provide separate compartmentsfor these bearings.

The other end of compressor chamber 32 is closed by end wall 38 whichcarries back end compressor bearings 40 and 42. Magnetic couplingcompartment 44 is separated from magnetic coupling compartment 46 bythin magnetic wall 48. Magnetic coupling disc 50 is mounted on the endof the compressor shaft to rotate therewith.

As it is seen in FIG. 3, the compressor shaft is coupled to the motorshaft so that the whole structure is driven by the motor. Magneticcoupling disc 52 faces disc 50 from compartment 46 so that magneticcoupling between the discs takes place through wall 48. Each of thediscs is a multiple magnet arranged so that the magnets can coupletogether to transmit torque. Disc. 52 is mounted on expander shaft 54which extends out of expander compartment 56 on low thermal loss hollowshafts. The companion expander shaft 58 carries the mating expanderrotor. The expander shafts are carried on bearings 60 and 62 incompartment 48 while the bearings 64 and 66 carry the other ends of theexpander shafts in compartment 68. Each of the bearings is provided witha seal between the bearing and the adjacent compressor or expanderchamber.

The rotors are designed such that the gas dynamics in the space betweenthe rotors holds them apart so that no timing gears are necessary toprevent mechanical contact between the rotors. By appropriate rotordesign, an expander 70 expanding air from one atmosphere to 0.5atmosphere while running at 10,000 rpm can operate without contact. In atest of that nature, the rotors were inspected after a one hour test andshowed no contact or wear.

As seen in FIG. 1, compressor 72 receives refrigerant gas at its rightend through suction line 74 and compresses it within chamber 32 todeliver it in pressure line 76. As indicated above, suitable pressuresare respectively 0.5 atmosphere in the suction line 74 and oneatmosphere in the pressure line 76. Suitable refrigerant gases depend onthe desired temperature, but include nitrogen, argon, carbon dioxide,neon, helium and hydrogen. The gas selected and the line pressuresdepend on the desired end temperature. Jacket 78 carries a coolant whichcarries off some of the heat of compression while after cooler 80 bringsthe refrigerant fluid in the pressure line 76 almost down to thetemperature of the coolant in coolant line 82. Counterflow heatexchanger 84 further cools the gas flowing in pressure line 76. Fromheat exchanger 84, the gas in pressure line 76 goes into the expanderchamber 56 at the left end which is the high pressure end of expander70. Rotation of the rotors in the expander chamber expand the gas intocold line 86 which is at the pressure of suction line 74. Heat load 88adds heat to the gas while the heat load 88 is cooled. From the heatload 88 the gas passes through counter flow heat exchanger 84 and backto the suction line 74 of compressor 72. The refrigeration cycle isdiscussed in more detail in patent application Ser. No. 894,677, filedApr. 10, 1978, the entire disclosure which is incorporated herein bythis reference. That application was abandoned and a continuationthereof was filed on June 12, 1980. The continuation applicationreceived Ser. No. 158,764 and is now U.S. Pat. No. 4,291,547 issuedSept. 29, 1981.

One of the specific improvements is the fact that there is a wall 48between the compressor and expander, with power transferred between thecompressor and expander through the magnetic coupling. Because of thepartition wall and the magnetic coupling there is no flow of gas betweenthe compressor housing and the expander housing and thus the pressure inthe adjacent bearing compartment is equal to the adjacent gas pressurein the chamber in the housing. In other words, there is no pressure dropwhich would drive lubricant from the bearing into the compressor orexpander chamber or which would drive refrigerant gas into the bearingsto drive lubricant out of the bearings.

In order to aid in bearing lubrication oil mist lubrication is provided.Radial blower 90, see FIG. 2 draws gas from compartment 68 together withthe oil suspended in the gas and discharges it out line 92 where it ispassed to compartment 44, see FIG. 3. From the top of compartment 44,line 94, carries the oil mist suspended in refrigerant gas back throughcompartment 68 to blower 90. When there is an oil mist of 1 to 5% pervolume of bearing lubrication oil to refrigerant gas, the blower 90maintains the oil in mist suspension as it passes through the lines andbearing compartments so that the bearings are continuously lubricated.The bearings 40, 42, 64 and 66 are lubricated by this circuit. From FIG.1, it can be seen that the ends of the corresponding compressor andexpander chambers adjacent these bearings are at the same pressure, thepressure of low pressure line 74 so that there is no pressure dropbetween the bearing compartment and the chambers so that the oil is notdriven one way or another.

Lubricant blower 96 is driven by the idler rotor in compressor 72. Itdraws refrigerant gas and suspended oil mist from the compartment ofbearings 34 and 36 and delivers it through line 97 to compartment 46where it serves to lubricate bearings 60 and 62. From that compartment,the mist is conveyed through line 98 to lubricate the bearing 22 andthence through line 100 to lubricate the other motor bearing 24. Fromthence, line 102 conveys the mist back to the compartment adjacentbearings 34 and 36 from whence it is drawn back into the blower 96 tocontinue its continuous closed cycle. The continuous closed cycle fromblower 96 distributes lubricant through bearing compartments which arethe same pressure as the adjacent gas of in this case 1 atmosphere sothere is no differential pressure across the bearings from adjacentcompartments. This reduces load on the bearing seals.

Another preferred embodiment of the screw compressor-expander cryogenicsystem of this invention is generally indicated at 104 in FIG. 4. Itcomprises a motor 106 which drives compressor 108 which has rotors 110and 112 of the type previously described. The rotor 110 is drivendirectly by motor 106 and rotor 112 is preferably driven by gas filmbetween the lobes and recesses on the rotors so that no mechanicalcontact takes place. The bearings of the motor, compressor and theexpanders may be lubricated by a circulating mist as described above.Magnetic coupling plates 114 and 116 operate through thin wall 118 sothat power is interchanged between compressor 108 and expander 120.Expander 120 has rotors 122 and 124 which are preferably separated bythe hydrodynamic gas film. Expander 120 is coupled to expander 126 bymeans of magnetic coupling plates 128 and 130 which operate through wall132. Expander rotors 134 and 136 expand gas in the chambers defined bythe lobes and recesses on the rotors and again the rotor 136 ispreferably separated from rotor 134 by a hydrodynamic gas film so thatno mechanical connection is necessary. All bearings are preferablylubricated by mist lubrication in order to provide a long life, and themist lubrication circuits are defined in such a manner that bearings atthe same pressure are connected together on the same mist lubricationcircuit and the bearings at different pressure are on separate mistlubrication circuits.

The cryogenic system could be arranged with the compressor 108 andexpanders 120 and 126 to provide two temperature levels of refrigerationby employing suitable counterflow heat exchanges between the variousflow streams. However, the screw compressor-expander cryogenicrefrigeration system illustrated in FIG. 4 is equipped with valves topermit its use with regenerator systems. Valve structure 140 isillustrative of valve structures 142 and 144 which are of similardesign, with the details of valve structure 140 illustrated in FIGS. 5and 6. Valve disc 146 is rotated by shaft 148 which is the shaft ofrotor 124. Openings 150 and 152 are formed through disc 146 and areillustrated as being approximately half circular. They are positioned tocover and uncover valve ports 154 and 156 respectively. When cyrogenicgas under pressure is provided in chamber 158 then the gas isalternately ported to the lines connected to valve ports 154 and 156.The configuration of openings 150 and 152 depends on the gas, therotative speed and the pressure, and the ports may be overlapped orunderlapped depending on particular conditions. As illustrated in FIG.6, they are slightly underlapped.

The ports 154 and 156 with their corresponding openings are in effectseparate valves which open at approximately opposite times. Forconvenience in the schematic diagram of FIG. 7 wherein the systemincorporating regenerators as generally indicated at 160, for theconvenience of schematic showing, the valving is separately shown.Similar valve ports 162 and 164 are shown in connection with compressor108 and valve ports 166 and 168 are shown in connection with expander126. As the compressor and expanders rotate the valves open and close toreverse flow in much of the system 160.

In FIG. 7 flow arrows are shown in connection with one position of thevalving, and it is understood that with a half turn of the valve discs,some of the flows are reversed. The direction of flow and the reversingflow will be discussed in connection with the following description ofthe schematic of FIG. 7 of the system 160. In considering the system,the flow arrows shown in FIG. 7 are for one portion of the flow cycle.In accordance with the usual regenerator practice, the valves arealternately operable so that flow direction through the regeneratorsreverses from time to time. At the time illustrated, compressor 108takes suction of refrigerant gas, compresses the gas and delivers itthrough heat exchanger 170 where heat is rejected to the ambient. Gasflow passes through open valve 162 and downward into the top ofregenerator 172 which is about 300° K. The down flowing gas is cooled bythe previously cooled regenerator packing. Part of the downward flow istaken out through line 174 where it enters expander 120 and is expandedtherein. The expanded gas passes out through open valve 156 and is insurface heat exchange with the body of regenerator 176 at heat exchanger178. This heat exchange is to stabilize the temperature. From heatexchanger 178, the fluid flow passes through heat exchanger 180 whichprovides refrigeration load at about 80° K. From the heat exchangerload, the fluid passes into regenerator 176 to rise therein.

Another portion of the downflowing cryogenic refrigerator gas stream inregenerator 172 is taken out of the regenerator at line 182 where itpasses into the high pressure end of the second stage expander 126.After it is expanded therein it passes out through valve 168 and thenthrough load heat exchanger 184 which is at about 40° K. From load heatexchanger the cryogenic refrigerant gas is delivered to the bottom ofregenerator 176 and it is moved upwardly therethrough joining the streamfrom load heat exchanger 180 to absorb heat from the regeneratorpacking. At the top of the regenerator the cryogenic fluid passesthrough line 186 and back to the suction of compressor 108.

At the next portion of the cycle, the valves 162, 156 and 168 closewhile the valves 164, 154 and 166 open to reverse the flow through theregenerators. By using a regenerator system, rather than a counterflowheat exchanger system, greater refrigeration efficiencies can beachieved.

This invention has been described in its presently contemplated bestmode and it is clear that it is susceptible to numerous modifications,modes and embodiments within the ability of those skilled in the art andwithout the exercise of the inventive faculty. Accordingly, the scope ofthis invention is defined by the scope of the following claims.

What is claimed is:
 1. A closed cycle refrigeration system comprising:ascrew compressor having a housing with a compressor chamber therein,said housing having inlet and outlet ends, an inlet and an outletadjacent the ends of the said housing; first and second helical rotorsrotatably mounted within said housing, said first and second helicalrotors respectively having intermeshing lobes and recesses configuredand fitted so that compression occurs in oil-free refringerant gaspassing through said compressor; an expander having an expander housingwith an expander chamber therein, said expander housing having inlet andoutlet ends, an inlet adjacent one end of said expander housing and anoutlet on the outlet end of said expander housing; first and secondhelical expander rotors rotatably mounted within said expander housing,said first and second helical expander rotors respectively havingintermeshing lobes and recesses configured and fitted so that gasexpansion takes place between said lobes and within said recesses inoil-free refrigerant gas upon rotation of said expander rotors in saidexpander housing; means interconnecting said inlet and outlet on saidcompressor housing and said inlet and said outlet on said expanderhousing for providing a closed cycle for the circulation of oil-freerefrigerant gas therein, for rejecting heat, and for receiving heat forproducing refrigeration upon rotation of said rotors; and a wall betweensaid compressor housing and said expander housing, magnetic drive meansconnected to rotate with one of said compressor rotors and magneticdrive means connected to rotate with one of said expander rotors formagnetically coupling said rotors to transfer power therebetween as theyrotate.
 2. The refrigerator system of claim 1 wherein a motor isconnected to one of said compressor rotors to drive said compressor andthe other of said compressor rotors is driven only by said motor drivenrotor.
 3. A refrigerator system comprising:a compressor housing having achamber therein, first and second interengaging helical compressorrotors in said compressor housing chamber arranged so that rotation ofsaid rotors causes oil-free gas compression in said compressor housingchamber; motor means connected to said first compressor rotor fordriving said first compressor rotor; an expander housing having anexpander chamber therein, first and second helical expander rotorsrotatably mounted in said expander chamber and interengaging so that assaid rotors rotate, oil-free gas is expanded in said expander chamber toproduce refrigeration; a wall between said compressor chamber and saidexpander chamber, first magnetic means positioned on one side of saidwall and connected to rotate with one of said helical compressor rotorsand magnetic means on the other side of said wall and magneticallycoupled with said first magnetic means and connected to rotate with oneof said helical expander rotors for transmitting power between saidcompressor rotor and expander rotor and for controlling the speed ofsaid expander rotor; and means interconnecting between said compressorchamber and said expander chamber and between a heat load and means forcirculating oil-free refrigerant gas and for rejecting heat forproviding refrigeration to said heat load.
 4. The refrigeration systemof claim 3 wherein said motor means is connected to drive one of saidcompressor rotors and said magnetic means is connected between saidmotor driven compressor rotor and one of said expander rotors.
 5. Therefrigeration system of claim 4 wherein the other of said expanderrotors is driven only by said rotor connected to said magnetic coupling.6. The refrigerator of claim 5 wherein the other of said compressorrotors is driven only by said motor driven compressor rotor.
 7. Therefrigeration of claim 4 wherein the other of said compressor rotors isdriven only by said motor driven compressor rotor.
 8. A closed cyclerefrigeration system comprising:a screw compressor having a housing witha compressor chamber therein, first and second helical compressor rotorsrotatably mounted within said housing, said first and second helicalcompressor rotors having intermeshing lobes and recesses configured sothat compression occurs in oil-free refrigerant gas passing through saidcompressor, an inlet and an outlet connected to said compressor chamber;first and second expanders connected to said compressor to exchangeenergy between said compressor and said expanders, each of saidexpanders having a housing with an expander chamber therein, each ofsaid expanders having first and second helical expander rotors rotatablymounted within their respective housing, said first and second helicalexpander rotors having intermeshing lobes and recesses configured sothat expansion occurs in oil-free refrigerant gas passing through saidexpanders, an inlet and an outlet connected to said expander chamber ofeach of said expanders; means connecting said inlet and outlet on saidcompressor housing and said inlets and outlets on said expanders forproviding a closed cycle for the circulation of oil-free refrigerant gastherein, for rejecting heat and for receiving heat for producingrefrigeration upon rotation of said rotors.
 9. The closed cyclerefrigeration system of claim 8 wherein there is a wall between each ofsaid housings and a magnetic coupling across said walls so that saidexpanders are magnetically coupled to rotate with said compressor totransfer energy therebetween as they rotate.
 10. The closed cyclerefrigeration system of claim 9 wherein said system includes motor meansto drive one of said compressor rotors and the other of said compressorrotors is driven only by said motor driven compressor rotor.
 11. Theclosed cycle refrigeration system of claim 8 wherein said systemincludes motor means to drive one of said compressor rotors and theother of said compressor rotors is driven only by said motor drivencompressor rotor.
 12. The cryogenic refrigerator system of claim 10wherein only one of said expander rotors is connected to said magneticcoupling to rotate with said compressor rotor and the other of saidexpander rotors is driven only by said rotor connected to said magneticcoupling.
 13. The cryogenic refrigerator system of claim 8 wherein onlyone of said expander rotors is connected to said magnetic coupling torotate with said compressor rotor and the other of said expander rotorsis driven only by said rotor connected to said magnetic coupling. 14.The cryogenic refrigerator system of claim 8 wherein said systemincludes reversing flow regenerators.
 15. The cryogenic refrigerator ofclaim 14 further including valves in said system for controlling fluidflow to and from said compressor and said expanders, said valvescontrolling a reversing flow to said regenerators for heat exchange withthe refrigerant gas and production of refrigeration.
 16. A cryogenicrefrigerator system comprising:a compressor housing having a chambertherein, first and second interengaging helical compressor rotorsrotatably mounted in said compressor chamber and arranged so thatrotation of said helical compressor rotors causes compression inoil-free refrigerant gas in said compressor chamber; an expander housinghaving an expander chamber therein, first and second helical expanderrotors rotatably mounted in said expander chamber and interengaging sothat as said expander rotors rotate, oil-free refrigerant gas isexpanded in said expander chamber; motor means connected to said firstcompressor rotor for driving said first compressor rotor; connectionmeans connecting one of said compressor rotors and said first expanderrotor for exchanging energy between said expander and said compressor;and a cryogenic system interconnecting said compressor and said expanderchambers and further including a regenerator, valve means, heatrejection means and heat receptor means for circulating oil-freerefrigerant gas for producing refrigeration at said heat receptor meansand rejecting heat to ambient at said heat rejection means when saidcompressor and said expander rotate and said valves are operated. 17.The cryogenic refrigerator system of claim 16 wherein there are firstand second expanders and each of said first and second expanders isconnected to expand gas in said cryogenic refrigeration system and eachof said first and second expanders is connected to a regenerator.